1. Field of the Invention
The present invention generally relates to a pulsive noise removing apparatus. More specifically, the present invention relates to a pulsive noise removing apparatus such as employed in an FM stereo receiver, for example, for removing a pulsive noise included in a stereo composite signal and for preventing a stereo pilot signal from being applied to a stereo demodulating circuit.
2. Description of the Prior Art
It is well-known that a pulsive noise such as a motor noise, an ignition noise generated by an automobile or the like could exert an influence upon normal reception by an FM receiver. Since such pulsive noise phase modulates the FM signal, the pulsive noise cannot be removed even by a limiter. Furthermore, since the frequency spectrum of such pulsive noise is distributed throughout a wide range from a low frequency to a high frequency, the same cannot be removed even by employment of a filter. Accordingly, a particular circuit for removing such pulsive noise has already been proposed and put into practical use.
FIG. 1 is a block diagram showing one example of an FM stereo receiver which constitutes the background of the invention. An FM stereo receiver comprises an antenna 1 for receiving FM broadcasting, a high frequency amplifier 3 for amplifying an FM signal received by the antenna 1, a local oscillator 5 for generating a local oscillation signal, a mixer 7 for mixing the amplified high frequency signal obtained from the high frequency amplifier 3 and the local oscillation signal obtained from the local oscillator 5 for converting the high frequency signal into an intermediate frequency signal, an intermediate frequency amplifier 9 for amplifying the intermediate frequency signal, and an FM detector 11 for demodulating the intermediate frequency signal amplified by the intermediate frequency amplifier 9 for providing a stereo composite signal. The stereo composite signal obtained from the FM detector 11 is applied to a stereo demodulating circuit 15 through a noise removing circuit 13. The stereo demodulating circuit 15 demodulates the stereo composite signal to provide a left signal and a right signal, which are applied to low frequency amplifiers 17 and 19. Speakers 21 and 23 are driven with the outputs from the low frequency amplifiers 17 and 19. The present invention is directed to an improvement in a pulsive noise removing apparatus such as inserted between the FM detector and the stereo demodulating circuit, as shown in FIG. 1.
One example of such pulsive noise removing apparatus is disclosed in Japanese Patent Publication No. 15710/1964 published for opposition Aug. 5, 1964, for example. The apparatus disclosed in the reference patent publication comprises a gate in a signal transfer path, which gate is interrupted for a given time period responsive to detection of a pulsive noise, whereby a pulsive noise is prevented from passing through the signal transfer path. A capacitor is also connected to the output point of the gate for the purpose of holding a level. The capacitor serves to maintain the level of a signal immediately before the gate is interrupted, whereby the signal is compensated with the signal maintained in the capacitor when the gate is interrupted. The pulsive noise is thus removed and distance of the signal during the gate interruption period is minimized. However, the above referenced Japanese Patent Publication No. 15710/1964 involves a problem that a stereo pilot signal is lost when the same is employed in an FM stereo receiver.
A pulsive noise removing apparatus which can prevent loss of a stereo pilot signal and can advantageously remove a pulsive noise has been proposed. One example is disclosed in U.S. Pat. No. 3,739,285 issued June 12, 1973. The apparatus disclosed in the referenced U.S. patent also comprises such gate and capacitor as described previously and a parallel resonance circuit is connected between the capacitor and the ground. The parallel resonance frequency of the parallel resonance circuit is selected to be the frequency of the pilot signal of the FM stereo broadcasting, say 19 kHz. Accordingly, the signal level immediately before the gate is interrupted is maintained in the capacitor and the pilot signal obtained from the parallel resonance circuit is applied to the stereo demodulating circuit. Thus, when the pulsive noise is detected, the gate is interrupted and the pulsive noise is prevented from being applied to the stereo demodulating circuit. Furthermore, when the gate is again closed, the continuity of the signal is maintained, inasmuch as the signal level has been maintained in the capacitor. At the same time, the pilot signal for the stereo demodulating circuit is prevented from being lost. Although the referenced United States patent can be advantageously utilized in an FM stereo receiver, still a further problem as set forth in the following is involved. More specifically, another series resonance circuit is formed with the capacitor and the parallel resonance circuit, whereby the signal being applied to the stereo demodulating circuit gives rise to distortion with such series resonance frequency. The series resonance frequency is necessarily smaller than the parallel resonance frequency and accordingly the same in an audible region and the distortion is outputted as a sound.
Therefore, the same assignee as the present invention previously proposed a noise removing apparatus directed to an improvement over the above referenced U.S. Pat. No. 3,739,285 and U.S. Pat. No. 4,066,845 issued Jan. 3, 1978 on the above described improvement. The last referenced U.S. patent teaches that an oscillation circuit is provided so that a stereo pilot signal is prevented by the oscillation circuit from being lost during the interruption period of the gate. A further pulsive noise removing apparatus is disclosed in Japanese Utility Model Laying-Open No. 106608/1978 laid-open Aug. 26, 1978. The referenced Japanese utility model laying-open discloses that a trap circuit is provided to a combination of the gate and capacitor so that the stereo pilot signal is removed by the trap circuit. Accordingly, a problem of losing the stereo pilot signal is eliminated; however, the interruption period of the gate is necessarily prolonged by the trap circuit, whereby discontinuity of the stereo composite signal is increased.
The same assignee as the present invention further proposed an improved noise removing apparatus in U.S. patent application, Ser. No. 133,932 and European patent application No. 80102251.8 (European Patent Publication No. 0018608). The last mentioned noise removing apparatus comprises two sets of a combination of a gate and a capacitor for maintaining the level. One of the gates receives a stereo composite signal, while the other receives a pseudo-pilot signal, and both are interrupted responsive to detection of a pulsive noise included in a composite signal. The outputs from the gate are applied to a signal synthesizing circuit such as an adding or subtracting circuit, for example. If and when a pulsive noise is included in the composite signal, the first gate is interrupted, whereby the pulsive noise is prevented from being applied to the adding or subtracting circuit, while the second gate is interrupted and the pseudo-pilot signal is also prevented from being applied to the adding or subtracting circuit. Accordingly, the signal maintained in the corresponding capacitor during the interruption period of the first and second gates is applied to the two inputs of the adding or subtracting circuit, whereby the stereo pilot signal is prevented from being obtained from the adding or subtracting circuit for that period. Meanwhile, when the first and second gates are rendered conductive, the stereo pilot signal is removed with the adding or subtracting circuit. In other words, the proposed noise removing apparatus simultaneously performs removal of a pulsive noise and cancellation of the pilot signal. Accordingly, an adverse influence upon the stereo pilot signal in the stereo demodulating circuit can be completely eliminated in an FM stereo receiver.
However, the proposed noise removing apparatus necessitates a combination of two sets of gates and capacitors, which makes complicated a circuit structure. In addition, since the signals from the two sets of circuits are utilized for offsetting the stereo pilot signals upon addition or subtraction thereof, both need have the same characteristics. Accordingly, diversification of the characteristics and capacitances of the transistors and capacitors constituting the two sets of circuits must be minimized and accordingly mass productivity is poor.
Furthermore, any of the above described prior art circuits comprises a gate implemented by a transistor, for example, in a transfer path of a stereo composite signal. Insertion of a gate in the signal path is one of the causes giving rise to distortion in the composite signal. The reason is that a transistor constituting the gate is not a completely linear device and hence involves non-linear portion. Accordingly, an apparatus including a gate inserted in the signal path necessitates another means for removing distortion caused by such non-linear portion. Without any means for removing such distortion, it becomes necessary not to insert a gate in a signal path; however, any apparatus of such structure which simultaneously performs removal of a pulsive noise and cancellation of pilot signal has not yet been proposed.